The present invention relates in general to communication systems, and is particularly directed to an aircraft data communication system having a plurality of wireless ground links that link respective aircraft-resident subsystems, in each of which a copy of its flight performance data is stored, with airport-located ground subsystems, each ground subsystem being coupled, in turn, by way of respective telco links to a remote flight operations control center, where flight performance data from plural aircraft parked at different airports may be analyzed and from which the uploading of in-flight data files may be directed by airline systems personnel.
Modern aircraft currently operated by the commercial airline industry employ airborne data acquisition (ADA) equipment, such as a digital flight data acquisition unit (DFDAU) as a non-limiting example, which monitor signals supplied from a variety of transducers distributed throughout the aircraft, and provide digital data representative of the aircraft""s flight performance based upon such transducer inputs. As flight performance data is obtained by the acquisition equipment, it is stored in an attendant, physically robust, flight data recorder (commonly known as the aircraft""s xe2x80x9cblack boxxe2x80x9d), so that in the unlikely event of an in-flight mishap, the flight data recorder can be removed and the stored flight performance data analyzed to determine the cause of the anomaly.
In a further effort to improve aircraft safety, rather than wait for an accident to happen before analyzing flight recorder data, the Federal Aviation Administration (FAA) has issued a draft advisory circular AC-120-XX, dated Sep. 20, 1995, entitled xe2x80x9cFlight Operational Quality Assurance Programxe2x80x9d (FOQA), which recommends that the airlines look at the information provided by the digital flight data acquisition unit at regular intervals.
One suggested response to this recommendation is to equip each aircraft with a redundant flight data recording unit having a removable data storage medium, such as a floppy disc. Such an auxiliary digital data recorder is intended to allow aircraft safety personnel to gain access to the flight performance data by physically removing the auxiliary unit""s data disc, the contents of which can then be input to an aircraft performance analysis data processing system for evaluation.
Although installing such a redundant flight data recording unit allows airline personnel to retrieve a copy of the flight performance data for subsequent evaluation, when considering the large volume of aircraft traffic experienced by major commercial airports, the above-proposed scheme is not only extremely time and manpower intensive, but is prone to substantial misidentification and aircraft/data association errors.
Other proposals, described in U.S. Pat. No. 5,359,446, are to use either a direct line-of-sight infrared link or a fiber optic cable to couple an on-board aircraft computer system with a ground-based computer system. Obvious drawbacks to these systems are the fact that not only do they employ complex and expensive components, but require that the aircraft be parked at the gate, so that the line-of-sight infrared transceivers or the fiber optic connection assemblies can be properly interlinked. As a consequence, neither of these types of systems is effective for use with commuter, cargo or military aircraft, which are customarily parked on an apron, rather than at a mating jetway, where such an optical link is to be provided.
In accordance with the present invention, the above-described objective of periodically analyzing flight performance data, without having to physically access a redundant unit on board the aircraft, is successfully addressed by means of a wireless ground data link, through which flight performance data provided by airborne data acquisition equipment is stored, compressed, encrypted and downloaded to an airport-resident ground subsystem, which forwards flight performance data files from various aircraft to flight operations control center for analysis. For purposes of providing a non-limiting example, in the description of the present invention, the data acquisition equipment will be understood to be a DFDAU.
For this purpose, an auxiliary data path is coupled from the DFDAU in parallel with the flight data recorder to a bidirectional radio frequency (RF) carrier-based ground data link (GDL) unit, that is installed in the avionics compartment of the aircraft. The GDL unit is operative to communicate with an airport-resident ground subsystem via the RF communications ground link infrastructure.
In accordance with a preferred embodiment of the invention, this wireless ground data link is implemented as a spread spectrum RF link. Numerous different frequency bands can be used in the present invention, as known to those skilled in the art. For example, several bands do not require an FCC site license. These bands include the 900 MHz, 2.4 GHz and 5.0 GHz bands. One particularly used band has been found to have a reasonably wide (about 100 MHz) unlicensed 2.4 to 2.5 GHz S-band segment, which provides global acceptance. Throughout the description, it should be understood that any frequency band can be used with the present invention. The description proceeds, however, with reference to the use of the 2.4 to 2.5 GHz band, which is known to have global acceptance. A benefit of spread spectrum modulation is its inherently low energy density waveform properties, which are the basis for its acceptance for unlicensed product certification. Spread spectrum also provides the additional benefits of resistance to jamming and immunity to multipath interference. Although throughout this description the use of direct sequence spread spectrum is described in detail, it should be understood that frequency hopping also can be used because of its advantages in spread spectrum, as well as chirp modulation. A forward error correction that does not use a pseudo ransom sequence can also be used. Well known M block modulation techniques can also be used. These types of modulations are well known to those skilled in the art.
A principal function of the GDL unit is to store a compressed copy of the (ARINC 717) flight performance data generated by the DFDAU and supplied to the aircraft""s flight data recorder. The GDL unit is also configured to store and distribute auxiliary information uploaded to the aircraft from a wireless router (as directed by the remote operations control center) in preparation for its next flight. The uploaded information may include audio, video and data, such as flight navigation information, and digitized video and audio files that may be employed as part of an in-flight passenger service/entertainment package. The GDL unit may also be coupled to an auxiliary printer that is ported to the GDL unit in order to enable an immediate hard copy of flight data information (e.g. exceedences of parameter data) to be provided to the crew immediately upon the conclusion of the flight.
Once an aircraft has landed and is within communication range of the ground subsystem, the wireless router receives flight performance data via the wireless ground data link from an aircraft""s GDL unit. It also supplies information to the aircraft in preparation for its next flight. The wireless router receives flight files from the aircraft""s GDL unit and forwards the files to an airport base station, which resides on the airport""s local area network (LAN).
The airport base station forwards flight performance data files from various aircraft by way of a separate communications path such as a telephone company (telco) land line to a remote flight operations control center for analysis. The airport base station automatically forwards flight summary reports, and forwards raw flight data files, when requested by a GDL workstation.
The flight operations control center, which supports a variety of airline operations including flight operations, flight safety, engineering and maintenance and passenger services, includes a system controller segment and a plurality of FOQA workstations through which flight performance system analysts evaluate the aircraft data files that have been conveyed to the control center.
Depending upon its size and geographical topography, an airport may include one or more wireless routers, that are installed within terminal buildings serving associated pluralities of gates, to ensure complete gate coverage. Redundant base stations may be utilized to assure high system availability in the event of a hardware failure. A large commercial airport exhibits the communication environment of a small city; consequently, it can be expected that radio communications between a respective wireless router and associated aircraft at gates will be subjected to multipath interference. In order to prevent the disruption of wireless router-GDL communications as a result of such a multipath environment, the wireless ground data link between each aircraft and a wireless router is equipped to execute either or both of a frequency management and an antenna diversity scheme.
Antenna diversity, which may involve one or more diversity mechanisms, such as spatial or polarization diversity, ensures that an aircraft that happens to be in a multi-path null of one antenna can still be in communication with another antenna, thereby providing full system coverage regardless of blockage. Frequency management is accomplished by subdividing a prescribed portion of the unlicensed radio frequency spectrum used by the system for GDLxe2x80x94wireless router communications into adjacent sub-band channels, and dynamically assigning such sub-band channels based upon the quality of the available channel links between a respective wireless router and a given aircraft. Such sub-channel assignments may involve downloading compressed and encrypted aircraft flight data over a first channel portion of the usable spectrum to the wireless router, and uploading information from a base station to the aircraft (e.g. video, audio and flight control data) from a wireless router over a second channel portion of the useable spectrum to the GDL on board the aircraft.
In a preferred embodiment, a respective wireless router employs a source coding system that achieves bandwidth reduction necessary to permit either multiple audio channels to be multiplexed onto the wireless transmit carrier to the GDL unit, video to be transmitted over a ground subsystem""s wireless router-to-GDL unit ground link frequency channel, or data files to be compressed to maximize system throughput and capacity during communications (uploads to or downloads from) the aircraft.
Cyclic Redundancy Check (CRC) coding is used for error detection only. When errors are detected at the wireless router, its transceiver requests a retransmission from the GDL unit, in order to guarantee that the copy of the flight performance data file downloaded from the GDL unit and forwarded from a wireless router is effectively error free.
In the uplink direction from the ground subsystem to the aircraft, the bit error rate requirements for transmitting passenger entertainment audio and video files are less stringent, and a forward error correction (FEC) and error concealment mechanism is sufficient to achieve a playback quality acceptable to the human audio/visual system. Also, since uploading an in-flight passenger audio/video file, such as a news service or entertainment program, may entail several tens of minutes (customarily carried out early in the morning prior to the beginning of airport flight operations), there is usually no additional time for its retransmission.
The wireless router transceiver includes a control processor which ensures robust system performance in the dynamically changing unlicensed spread spectrum interference environment of the ground data link by making decisions based on link signal quality, for the purpose of setting transmit power level, channel frequency assignment, and antenna selection. The ground subsystem processor also initiates a retransmission request to an aircraft""s GDL unit upon detection of a bit error in a downlinked flight performance data packet.
Before requesting retransmission of a flight data packet, the wireless router""s transceiver measures the signal quality on the downlink channel portion of the ground data link. The transceiver in the wireless router assesses the measured link quality, increases its transmit power level as necessary, and requests a retransmission of the packet containing the bit error at a higher transmit power level. It then initiates a prescribed frequency management protocol, to determine if another channel portion of the GDL link would be a better choice. If a higher quality channel is available, both transceivers switch over to the new frequency. The flight performance data packet containing the bit error is retransmitted until it is received error free at the wireless router.
Because the invention operates in an unlicensed portion of the electromagnetic spectrum, it can be expected to encounter other unlicensed communication products, such as employed by curbside baggage handling and ticketing, rental car and hotel services, etc., thereby making the communication environment unpredictable and dynamically changing. To solve this problem, the present invention employs a frequency management scheme, which initially determines the optimum operating frequency and automatically changes to a better quality frequency channel when the currently established channel suffers an impairment.
The spread spectrum transceiver in each of an aircraft""s GDL unit and an associated airport wireless router includes a frequency agile spread spectrum transmitter, a frequency agile spread spectrum receiver and a frequency synthesizer. In addition to being coupled to an associated control processor, the spread spectrum transmitter is coupled to an adaptive power control unit and an antenna diversity unit. Such a power allocation mechanism makes more efficient use of available power sources, reduces interference, and makes more efficient use of the allocated frequency spectrum. The control processors at each end of the wireless ground link execute a communication start-up protocol, through which they sequentially evaluate all of the available frequency channels in the unlicensed 2.4-2.5 GHz S-band segment of interest and assess the link quality of each of these channels.
Each wireless router transceiver sequentially and repeatedly sends out a probe message directed to any of the GDL units that are within the communication range of gates served by that wireless router, on each of all possible frequency channels into which the 2.4-2.5 GHz S-band spread spectrum bandwidth has been divided. Each GDL unit within communication range of the wireless router returns a response message on each frequency channel, and indicates which frequency is preferred, based upon the signal quality assessment and measured signal quality by its communication processor.
The wireless router control processor evaluates the responses from each of the GDL units, selects the frequency of choice, and then notifies the GDL units within communication range of its decision. This process is periodically repeated and is executed automatically in the event of a retransmission request from a GDL unit.
As described earlier, in an environment such as a large commercial airport, a common cause of reduced signal quality is multipath interference resulting from sudden attenuation in the direct path between the transmitters and the receivers in the wireless router and aircraft, in conjunction with a delayed signal arriving at the receiver from a reflected path. This sudden attenuation in the direct path between the aircraft and the wireless router can result in the destructive summation of multiple paths at the antenna in use, resulting in a severe signal fading condition. The nature of multipath is such that switching to a second spatially separated or orthogonally polarized antenna can result in a significant improvement in link performance. Since the wireless networking environment of an airport is one in which objects are likely to be moving between the wireless router and the aircraft, and one of the platforms (the aircraft) is mobile, antenna diversity can make the difference between reliable and unreliable system performance.
Pursuant to the invention, upon the occurrence of a prescribed reduction in link quality, an antenna diversity mechanism is employed. Such a mechanism may involve the use of separate transceivers (each having a respective antenna), or an antenna diversity unit that switches between a pair of spatially separated or orthogonally polarized antennas. Link performance is evaluated for each antenna in real time, on a packet-by-packet basis, to determine which antenna provides the best receive signal quality at the wireless router.
Signal quality is continually measured at the receiver demodulator output and reported to the control processor. Should there be a sudden degradation in link signal quality, the wireless router control processor switches over to the other antenna. If the degradation in signal quality cannot be corrected by invoking the antenna diversity mechanism, such as by switching antennas, the wireless router has the option of increasing the transmit power level at both ends of the link to compensate for the reduction in link quality and/or execute the frequency management routine to search for a better operating channel. In the wireless router""s broadcast mode, the same signal can be transmitted from both antennas in order to assure reliable reception at all aircraft, regardless of changing multipath conditions.